The present invention relates to a method for producing an electrode for a non-aqueous electrolyte battery, and more particularly to a method for producing a non-aqueous electrolyte battery electrode which includes an electrode active material layer containing an active material having a composition of LixNiyMzO2 (where x satisfies 0.8 less than x less than 1.5, y+z satisfies 0.8 less than y+z less than 1.2, z satisfies 0xe2x89xa6z less than 0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe) and having excellent battery characteristics and excellent physical properties of the active material layer.
Recent development in the electronic field is remarkable, and reduction in size and weight of devices such as video cameras, liquid crystal cameras, portable telephones, laptop computers, and word processors is now taking place. There are increasing needs for development of batteries with reduced size and weight and having a high energy density as power sources for these electronic devices.
Conventionally, lead batteries or nickel-cadmium batteries have been used for these electronic devices. These conventional batteries cannot sufficiently meet the needs for reduction in size and weight and achievement of a high energy density.
As batteries that meet these demands, development of non-aqueous electrolyte secondary batteries employing metal lithium or a substance capable of being doped and undoped with lithium as a negative electrode has been carried out, whereby those using a lithium cobalt oxide (LiCoO2) as a positive electrode material are already in practical use. This battery has a property of having a high voltage and a high energy density as compared with the conventional small secondary batteries. Therefore, they are greatly expected as a power source for driving a cordless devices, so that secondary batteries which are smaller and lighter than the conventional batteries can be fabricated.
Also, in order to achieve further reduction in size and weight and to obtain a further high energy density, development and research of active materials etc. is eagerly conducted, and a lithium nickel composite oxide LiNiO2 is proposed as a positive electrode active material.
Here, in the electrode of a non-aqueous electrolyte battery, an electrically conductive agent is used because the active material has a poor electric conductivity except for some active materials.
For example, Japanese Laid-open Patent Publication No. 1-105,459/1989 discloses a non-aqueous electrolytic solution secondary battery including a positive electrode containing LiMn2O4 and graphite as major components, a negative electrode, and a non-aqueous electrolytic solution, wherein a graphite content in the total amount of the above LiMn2O4 and the graphite is 8 to 22 wt %. This means that, in using graphite as an electrically conductive agent, effects are not produced unless the graphite is added in a large amount.
Japanese Laid-open Patent Publication No. 4-215,252/1992 discloses use of a flake graphite as an electrically conductive agent for a positive electrode in a non-aqueous electrolyte secondary battery. In this way, although there are disclosures on electrically conductive agents made of graphite, the prior art fails to disclose a method of preparing a slurry.
Generally, the larger the amount of the electrically conductive agent is, the easier it is to draw out the efficiency of the active material. However, if a large amount of the electrically conductive agent is incorporated in the electrode, the amount of the active material per unit volume will decrease, so that as a result the capacity as a battery will decrease. Therefore, attempts are made to reduce the amount of the electrically conductive agent while drawing out the efficiency of the active material.
However, according to the conventional methods of preparing an electrode mixture-coating material, a large amount of the electrically conductive agent must be added in the electrode mixture in order to obtain a sufficient electrode conducting property. As a result, there arises a problem that the charge/discharge capacities per unit electrode volume or per unit electrode weight decreases.
Also, non-aqueous electrolyte secondary batteries get deteriorated in that their capacity decreases in accordance with their repeated use. One of the factors that cause such a deterioration of the batteries is considered to be that the contact of the active material and the electrically conductive agent in the electrode gets poorer, making it difficult to draw out an electricity to the outside.
In view of these facts, it is desirable to maintain a good contact of the active material and the electrically conductive agent in the electrode while reducing the amount of the electrically conductive agent of graphite as much as possible.
Accordingly, the purpose of the present invention is to solve the above-mentioned problems of the prior art and to provide a method for producing an electrode for a non-aqueous electrolyte battery with improved charge/discharge characteristics such as a discharge capacity and a charge/discharge cycle life and with improved physical properties of a coating film.
The present inventors have made an eager research and found out that an electrode having good charge/discharge characteristics and stable physical properties of the coating film can be obtained even with a small amount of an electrically conductive agent by wet-kneading a slurry containing an active material and the electrically conductive agent and then performing a special dispersion process in preparing an electrode active material mixture-coating material, thereby completing the present invention.
Accordingly, the present invention provides a method for producing an electrode for a non-aqueous electrolyte battery by preparing an electrode active material mixture-coating material containing at least an active material having a composition of LixNiyMzO2 (where x satisfies 0.8 less than x less than 1.5, y+z satisfies 0.8 less than y+z less than 1.2, z satisfies 0xe2x89xa6z less than 0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe), an electrically conductive agent and a binder, and by applying said active material mixture-coating material on a collector, said method comprising the steps of wet-kneading a slurry containing at least said active material, said electrically conductive agent, said binder and an organic solvent, and then performing a dispersion process on said wet-kneaded slurry by means of a narrow-gap-type dispersing apparatus to prepare the mixture-coating material.
In the present invention, a concentration of solid components in the slurry at the wet-kneading step is preferably 70 to 90 wt %.
Also, in the present invention, the wet-kneading step is performed preferably in an atmosphere of air, nitrogen gas, or a rare gas having a moisture dew point of not more than xe2x88x9220xc2x0 C.
According to the present invention, since the slurry is wet-kneaded and then subjected to the dispersion process by means of the narrow-gap-type dispersing apparatus in preparing the active material mixture-coating material, a sufficient electric conductivity of the electrode can be obtained with a small amount of the electrically conductive agent. As a result, an amount of the active material per unit electrode volume can be increased and large charge/discharge capacities per unit electrode volume can be obtained. Also, physical properties of a coating film can be stabilized.
The active material to be used in the present invention is a lithium composite oxide represented by the general formula: LixNiyMzO2 (where x satisfies 0.8 less than x less than 1.5, y+z satisfies 0.8 less than y+z less than 1.2, z satisfies 0xe2x89xa6z less than 0.35, and M is at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe).
In the present invention, an average particle diameter of the above-mentioned lithium composite oxide is preferably within the range of 1.0 to 30 xcexcm, more preferably within the range of 2.0 to 20 xcexcm. A half-value width at the average particle diameter of its particle size distribution is preferably within the range of 2.0 to 50 xcexcm. The particle size distribution is measured by the laser diffraction/scattering method.
An example of a method for producing such a lithium composite oxide is, for example, a process in which a basic metal salt and an alkaline water-soluble lithium compound containing respectively an anion that volatilizes at the time of calcination of LiMetal3+O2 (where the Metal contains Ni as a major component and further contains at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe) are allowed to react in an aqueous medium to obtain a slurry, which is then dried and calcined.
The basic metal salt is represented by the general formula: Metal2+(OH)2-nk(Anxe2x88x92)kxc2x7mH2O. Here, the Metal2+ is an ion containing Ni as a major component and possibly containing at least one element selected from Co, Mg, Ca, Sr, Al, Mn and Fe. Anxe2x88x92 represents an anion with n valences (where n=1 to 3) such as a nitrate ion, a chloride ion, a bromide ion, an acetate ion, or a carbonate ion. Further, k satisfies 0.03xe2x89xa6kxe2x89xa60.3; and m satisfies 0xe2x89xa6m less than 2.
The basic metal salt represented by the above-mentioned formula can be produced by adding to an aqueous solution of Metal2+ an alkali of about 0.7 to 0.95 equivalent, preferably about 0.8 to 0.95 equivalent, relative to the Metal2+, and reacting them under a reaction condition of about 80xc2x0 C. or less, and then maturing the reaction product at a temperature of 40xc2x0 C. to 70xc2x0 C. for 0.1 to 10 hours, followed by washing with water to remove the by-products. The alkali to be used in the reaction may be a hydroxide of an alkali metal such as sodium hydroxide, a hydroxide of an alkali earth metal such as calcium hydroxide, an amine, or the like.
A basic metal salt selected from the compounds represented by the above-mentioned formula and one or more lithium compounds selected from lithium hydroxide, lithium carbonate, hydrates thereof, and the like are allowed to react in water at a concentration in the range of 5 to 25 wt % and at a temperature in the range from room temperature to 100xc2x0 C. to obtain a slurry, which is then subjected to spray drying for improvement of uniformity in the shape of the composition to be obtained.
The lithium composite oxide can be obtained by subjecting the dried product to a thermal treatment for calcination in an oxidizing gas atmosphere containing air, oxygen, ozone, or the like in a temperature range of about 700 to 1000xc2x0 C. for about 0.1 to 20 hours.
Another example of a method for producing a lithium composite oxide to be used in the present invention is a process that uses a water-soluble lithium compound and a basic metal carbonate obtained from a water-soluble metal compound.
The water-soluble metal compound to be used in this process is a nitrate, a sulfate, a metal chloride, or the like. This water-soluble metal compound may contain a nickel compound as a major component and may be mixed with a given amount of another water-soluble metal compound so that at least one element selected from Co, Mg, Ca, Sr. Al, Mn and Fe may be blended therewith.
The basic metal carbonate may be obtained by filtrating and drying a precipitate obtained by allowing a mixture of the above-mentioned water-soluble metal compounds to react with a compound selected from the group consisting of an alkali carbonate, an alkali bicarbonate, ammonium carbonate and ammonium bicarbonate in water, or a precipitate obtained by allowing sodium hydroxide to be present for reaction in the above-mentioned reaction system. In this case, in order to produce a good precipitate, it is preferable to use a little excessive amount of the carbonate, and also it is important to control the stirring condition so as to control the specific surface area of the precipitate.
To the basic metal carbonate thus obtained, a powder of a water-soluble lithium compound such as lithium carbonate or lithium hydroxide is added at a desired ratio of the metal to Li. The resultant mixture in a powder state is first heated to 300 to 500xc2x0 C. in the presence of an inert gas or an oxygen-containing gas. This heating allows only the decomposition of the basic metal carbonate to proceed, whereby carbonic acid gas in the crystal structure is released. This heating is continued until the generation of the carbonic acid gas substantially stops so as to convert all of the basic metal carbonate into a metal oxide having numerous fine pores.
After the generation of carbonic acid gas substantially stops, the temperature is further raised to allow the molten water-soluble lithium compound to penetrate into the fine pores of the metal oxide, whereby the two compounds will be in an extremely close contact. At this moment, the resultant product is calcined at a temperature of 700 to 900xc2x0 C. in the presence of oxygen gas or an air rich in oxygen, whereby Ni is turned from bivalent to trivalent to produce a Li composite oxide.
Here, the larger the specific surface area of the basic metal carbonate to be used is (for example, more than 100 m2/g), the more preferable it is, because gas discharge and generation of fine pores after preliminary calcination will be more efficiently performed.
The electrically conductive agent to be used in the present invention is not specifically limited and may be any electron-conducting material that does not undergo a chemical change. Examples of the electrically conductive agent typically include graphites such as natural graphite (scaled graphite, scale-like graphite and others) and artificial graphite, carbon blacks such as acetylene black, Ketjen black, channel black, furnace black, and thermal black, electrically conductive fibers such as carbon fiber, carbon fiber grown in gaseous phase, and metal fiber, and metal powders such as copper, nickel, aluminum, and silver. Among these, scale-like graphite is preferable.
Specifically, the electrically conductive agent may be one of the LF series manufactured by Chuuetsu Kokuen Kogyosho Co., Ltd., the UFG series manufactured by Showa Denko Co., Ltd., the KS series manufactured by LONZA Co., Ltd., MICROCARBO-G series manufactured by Kansai Netsukagaku Co., Ltd., Ecoscarbons manufactured by Ecos Giken Co., Ltd., and others.
The amount of the electrically conductive agent to be blended is preferably 1 to 15 wt %, more preferably 2 to 10 wt %, in a dried coating film, although it depends on the specific surface area of the active material and the like.
The binder to be used in the present invention is one or a mixture of two or more of thermoplastic resins or polymers having a rubber elasticity. Examples of the binders to be used include fluorine-type polymers, polyvinyl alcohol, carboxymethyl cellulose, hydroxypropyl cellulose, regenerated cellulose, diacetyl cellulose, polyvinyl chloride, polyvinyl pyrrolidone, polyethylene, polypropylene, EPDM, sulfonated EPDM, SBR, polybutadiene, polyethylene oxide, and the like.
Among these, the fluorine-containing polymer preferably has an atomic ratio of fluorine atoms/carbon atoms in the range of 0.75 to 1.5, more preferably in the range of 0.75 to 1.3. If this value is larger than 1.5, it tends to be difficult to obtain a sufficient capacity of the battery. On the other hand, if it is smaller than 0.75, the binder is likely to be dissolved in the electrolytic solution.
Examples of such fluorine-containing polymers include polytetrafluoro-ethylene, polyvinylidene fluoride, vinylidene fluoride-ethylene trifluoride copolymers, ethylene-tetrafluoroethylene copolymers, propylene-tetrafluoroethylene copolymers, and the like. A fluorine-containing polymer with its hydrogen in the main chain substituted by an alkyl group(s) may be used as well.
Among these, those showing a selective solubility (having a low solubility in the electrolytic solution and being soluble in some solvents). For example, vinylidene fluoride-type polymers are only slightly soluble in a carbonate-type solvent that is used as the electrolytic solution or the like, but are soluble in a solvent such as N,N-dimethylformamide or N-methylpyrrolidone.
The amount of the binder to be added is preferably 1 to 15 wt %, more preferably 2 to 10 wt %, in a dried coating film, although it depends on the specific surface areas and the particle size distributions of the active material and the electrically conductive agent, the strength of the intended electrode, and the like.
The solvent to be used for the preparation of the electrode active material mixture-coating material is not specifically limited and may be a general organic solvent. Examples of the organic solvents include saturated hydrocarbons such as hexane, aromatic hydrocarbons such as toluene and xylene, alcohols such as methanol, ethanol, propanol, and butanol, ketones such as acetone, methyl ethyl ketone, methyl isobutyl ketone, diisobutyl ketone, and cyclohexanone, esters such as ethyl acetate and butyl acetate, ethers such as tetrahydrofuran, dioxane, and diethyl ether, amides such as N,N-dimethylformamide, N-methylpyrrolidone, and N,N-dimethylacetamide, halogenated hydrocarbons such as ethylene chloride and chlorobenzene, and the like. Among these, ketone-type and amide-type solvents are preferable because they can dissolve the fluorine-containing polymers. These solvents may be used either alone or as a mixture of two or more thereof.
In the present invention, a slurry containing the active material, the electrically conductive agent, the binder and the organic solvent as mentioned above is wet-kneaded and then subjected to a dispersion process by means of a narrow-gap-type dispersing apparatus to prepare a mixture-coating material. Through these steps, it is possible to remove efficiently a secondary agglomeration or a stack of the electrically conductive agent, whereby a sufficient electrically conductive effects are produced even if only a small amount of the electrically conductive agent is added. Also, the physical properties of the coating film is stabilized.
For example, an explanation will be given on a case where graphite is used as the electrically conducive agent. Since graphite has a cleavage property, graphite having a large particle diameter can be cleft to increase its specific surface area with little decrease in the particle diameter. However, graphite is liable to form a stack and, if a strong shear stress or the like is applied in a dry process to loosen the stack, it is likely to be stacked to a greater extent. For example, if an attempt is made to crush graphite with a ball mill or the like, there arises a problem that the graphite is liable to be stacked, although it can be crushed. Therefore, it was difficult in the prior art to cleave the stack without breaking the graphite particles. According to the operation of the present invention, the stack of graphite can be effectively removed, and sufficient electrically conducting effects can be obtained even with addition of only a small amount of graphite.
Further, an explanation will be given on a case where acetylene black is used as the electrically conductive agent. Acetylene black is liable to form large secondary agglomerates. By a conventional dry process or a conventional dispersion method, it is not easy to loosen the agglomeration of acetylene black particles, so that a large amount of acetylene black had to be added in the electrode mixture-coating material in order to obtain sufficient electrical conductivity. According to the operation of the present invention, it is possible to loosen the secondary agglomerates of acetylene black, whereby sufficient electrically conducting effects can be obtained even with addition of only a small amount of acetylene black.
In the present invention, the wet-kneading is conducted in a kneading apparatus such as a kneader, a planetary mixer, a two-roll rolling mill, a three-roll rolling mill, a Banbury mixer, and the like. The kneader is an apparatus that gives a shear between a rotating blade and a kneading tank. The kneader may be of a single-process type or a continuous-process type. Either of these two types may be used.
Examples of single-process type kneaders include an open-type kneader, pressure-type kneader, and the like. Since the open-type kneader is open at its top, voids are generated, so that it is rather unsuitable for achieving the object of the present invention. The pressure-type kneader can produce the effects of the present invention in the most efficient manner because the voids can be controlled to the minimum. Also, the kneader of continuous process type does not decrease the effects of the present invention because it has a feeding structure, although it slightly generates voids as compared with the pressure type. The kneader of continuous process type is preferable because it can carry out the steps continuously including the preprocesses such as preliminary mixing and the postprocesses such as kneading, dilution, and dissolution.
Examples of the rotating blade to be used in such a kneader include a xcexa3-type blade, a Z-type blade, a cam-type blade, a roller-type blade, an S-type blade, a fishtail-type blade, and a Banbury-type blade.
Examples of the kneader include single-type kneaders such as an MS-type pressure-type kneader manufactured by Moriyama Co., Ltd., continuous kneaders such as KRC kneaders manufactured by Kurimoto, Ltd., and extruders manufactured by a Fuji Powdernel, Kobe Seitetsusho, Toshiba Machine Co., Ltd., and further, small-type kneaders such as a desktop-type kneader manufactured by Irie Shokai Co., Ltd., a kneader manufactured by Takabayashi Rika Co., Ltd., a Laboplastomill and a Blavender manufactured by Toyo Seiki Co., Ltd.
A concentration of solid components in the slurry at the time of the wet-kneading step is preferably within the range of 70 to 90 wt %. If the concentration is too larger or too smaller than the above range, it is not possible to obtain a sufficient shear force.
Also, the wet-kneading step is preferably carried out in an atmosphere of air, nitrogen gas, or a rare gas having a moisture dew point of not more than xe2x88x9220xc2x0 C. If the slurry is wet-kneaded in such an atmosphere, it is possible to prevent the above-mentioned LixNiyMzO2, which is the active material, from undergoing a decomposition reaction, so that the charge/discharge capacities do not decrease.
A slurry temperature at the time of the wet-kneading step is preferably within the range of 30 to 90xc2x0 C. If the slurry temperature is below 30xc2x0 C., it will be difficult to obtain a sufficient shear force. On the other hand, if the slurry temperature exceeds 90xc2x0 C., the organic solvent in the slurry is likely to evaporate, thereby changing the concentration of solid components in the slurry.
By means of the above-mentioned kneading apparatus and the kneading operation, the slurry containing the active material, the electrically conductive agent, the binder and the organic solvent is sufficiently kneaded. The kneaded slurry is then diluted to have a suitable viscosity and dispersed in a narrow-gap-type dispersing apparatus.
The narrow-gap-type dispersing apparatus to be used in the present invention may be a Gaulin homogenizer, a high-speed stone mill, an ultradisper, or the like. Any of the above-mentioned dispersing apparatus gives a shear by passing a kneaded coating material through a narrow gap. By performing such a dispersing process, fine portions such as stacks of graphite or secondary agglomerates of acetylene black can be dispersed to a higher degree. Therefore, the maximum effects of the electrically conductive agent can be obtained with addition of the minimum amount of the electrically conductive agent.
Also, in accordance with the needs, a final adjustment may be made by dispersion in a stirring mixer, such as a hyper mixer or a dissolver, so that the electrode active material mixture-coating material may meet the condition of a coating apparatus.
The prepared mixture-coating material is applied onto a collector and dried. The collector is not specifically limited and may be any electron-conducting substance that does not undergo a chemical change when it is formed into a battery. For example, the collector may be formed of aluminum, an aluminum alloy, nickel, stainless steel, titanium, or the like. Besides these metals, aluminum or stainless steel whose surface is treated with carbon, nickel, or titanium may be used as well. Especially, aluminum or an aluminum alloy is preferable. The collector may have a foil-like, film-like, net-like, porous, foamed, or the like shape. The thickness of the collector is 1 to 100 xcexcm, preferably 1 to 50 xcexcm.
The electrode active material mixture-coating material is applied onto the collector by an ordinary well-known coating method such as the reverse roll method, direct roll method, blade method, knife method, extrusion nozzle method, curtain method, gravure roll method, bar coat method, dip method, kiss coat method, squeeze method, and the like. Among these, it is preferable to adopt the extrusion nozzle method, whereby a good surface state of the coating layer can be obtained by suitably selecting the solvent composition of the mixture and the drying condition so that the coating material is applied onto the collector at a coating speed of 5 to 100 m/min.
The drying temperature is preferably 30 to 150xc2x0 C., more preferably 50 to 140xc2x0 C. If the drying temperature is less than 30xc2x0 C., the solvent will not be sufficiently dried. On the other hand, if the drying temperature exceeds 150xc2x0 C., the binder may not be distributed uniformly on the surface of the electrode because the evaporation speed of the solvent is too high, thereby leading to deterioration in the electrode properties.
Here, the thickness, length, and width of the coating layer is determined by the final size of the battery to be obtained. The thickness of the coating layer is preferably adjusted by the ordinarily-adopted calendar processing after the coating step. The processing pressure is preferably 0.2 to 10 t/cm, and the processing temperature is preferably 10 to 150xc2x0 C.